This allows for consistent messaging about each status to be shown in different parts of the system. Previously these strings were hardcoded in the loading screen html so were inaccessible elsewhere.
This adds the loadAsynchronously() method to ArbitraryDataReader, in addition to the existing loadSynchronously() method.
When requesting a website in a browser, previously the building of the resource's layers would be done synchronously in the API handler. This understandably caused many issues, so the building is now done asynchronously by a dedicated thread. A loading screen is shown in its place which auto refreshes every second until the build has completed.
